Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are overlapping neurodegenerative diseases with no effective treatment. Success in developing a treatment will require a well-orchestrated effort that addresses multiple aspects of the drug discovery process, including target identification and validation, as well as the identification of biomarkers to assess efficacy of potential therapies in clinical trials. These endeavors have been hampered by an incomplete understanding of FTD and ALS pathogenesis. However, with the discovery that a GGGGCC repeat expansion in C9ORF72 is the most common genetic cause of FTD and ALS (DeJesus-Hernandez et al., 2011; Renton et al., 2011), a new therapeutic target has come to light.
Two putative pathomechanisms of “c9FTD/ALS” involve RNA transcribed from the expansion. First, these transcripts (termed r(GGGGCC)exp) may cause toxicity through the formation of nuclear RNA foci that sequester various RNA-binding proteins [for review, see (Gendron et al , 2014)]. Second, r(GGGGCC)exp undergoes repeat associated non-ATG (RAN) translation producing “c9RAN proteins” that form neuronal inclusions throughout the central nervous system (Ash et al., 2013; Mori et al., 2013b). Consequently, neutralizing or degrading r(GGGGCC)exp holds promise as a therapeutic approach for c9FTD/ALS. Indeed, antisense oligonucleotides to C9ORF72 transcripts suppress features associated with the repeat expansion in human induced pluripotent stem cell-derived neurons (Donnelly et al., 2013; Sareen et al., 2013). In light of pharmacological advantages, small molecules may offer an attractive option for targeting r(GGGGCC)exp. Capitalizing on our findings that r(GGGGCC)n adopts a hairpin structure in addition to a G-quadruplex one, we designed small molecules able to bind r(GGGGCC)exp and to significantly decrease RAN translation and foci formation in cultured cells expressing r(GGGGCC)66 (SEQ ID NO:1) and in induced neurons (iNeurons) directly converted from fibroblasts of C9ORF72 repeat expansion carriers. These findings indicate that designer small molecules targeting r(GGGGCC)exp may prove promising as a c9FTD/ALS therapeutic. Furthermore, since we found that poly(GP) c9RAN proteins are detected in c9ALS cerebrospinal fluid (CSF), poly(GP) proteins may serve as a pharmacodynamic biomarker to assess efficacy of potential therapies that target r(GGGGCC)exp.